Support in-place upgrades of new minio binary and releases. (#4961)

This PR allows 'minio update' to not only shows update banner
but also allows for in-place upgrades.

Updates are done safely by validating the downloaded
sha256 of the binary.

Fixes #4781

vendor/github.com/inconshreveable/go-update/internal/binarydist/doc.go

@@ -0,0 +1,24 @@
+// Package binarydist implements binary diff and patch as described on
+// http://www.daemonology.net/bsdiff/. It reads and writes files
+// compatible with the tools there.
+package binarydist
+
+var magic = [8]byte{'B', 'S', 'D', 'I', 'F', 'F', '4', '0'}
+
+// File format:
+//   0       8    "BSDIFF40"
+//   8       8    X
+//   16      8    Y
+//   24      8    sizeof(newfile)
+//   32      X    bzip2(control block)
+//   32+X    Y    bzip2(diff block)
+//   32+X+Y  ???  bzip2(extra block)
+// with control block a set of triples (x,y,z) meaning "add x bytes
+// from oldfile to x bytes from the diff block; copy y bytes from the
+// extra block; seek forwards in oldfile by z bytes".
+type header struct {
+	Magic   [8]byte
+	CtrlLen int64
+	DiffLen int64
+	NewSize int64
+}